🐛 Bug Report

getMockImplementation is a public method of the mock function api:
https://github.com/facebook/jest/blob/244f7dc7310de24b4892fac55113f8a06f24d947/packages/jest-mock/src/index.ts#L103
But it is nod documented at
https://github.com/jwbay/jest/blob/master/docs/MockFunctionAPI.md

Also, I believe we can better type it as we know the possible return types